1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus including a plurality of image forming units.
2. Description of the Related Art
In general, when a color image is printed on a recording medium such as paper, an electrophotographic color image forming apparatus using a plurality of developers (for example, toners) of different colors is used. In recent years, there are cases in which a white image is printed on colored paper (for example, black paper), a transparent film used for an overhead projector (OHP), and the like. In such cases, image forming apparatuses including an image forming unit which uses a white toner are utilized. See Patent reference 1, Japanese patent application publication No. 2014-32280, for example.
However, in a case where an image is formed by an image forming apparatus using a toner (for example, white toner) having characteristics different from those of color toners (for example, black color toner, cyan color toner, magenta color toner, and yellow color toner) generally used in electrophotographic color image forming apparatuses, “fogging toner” increases and therefore a phenomenon called “fogging” tends to occur, thereby lowering the image quality. Furthermore, “fogging toner” is a low-charge-amount toner (i.e., a toner with a small absolute value of charge amount) that could cause fogging and a toner charged to a polarity opposite to a polarity to which the toner should be normally charged. “Fogging” is a phenomenon that the toner having a lower charge amount than the normally charged toner or the toner charged to a polarity opposite to a polarity to which the toner should be normally charged adheres to a background of the image (that is, a non-image area).